Crossposting from here: https://community.esri.com/t5/arcgis-online-questions/arcgis-online-new-map-viewer-doesn-t-support-w...
Please provide support for WMS legends in the New Web Map Viewer spec. It works if you add the legendURL in manually, but the webmap by default doesn't add it, and there is no gui to add it yourself.
Thanks,
Marc